1. Field of Invention
The present invention relates to a memory structure and a method for fabricating the same. More particularly, the present invention relates to a structure of a mask programmable read-only memory (Mask ROM) and a method for fabricating the same.
2. Description of Related Art
A Mask ROM generally comprises a substrate, a plurality of buried bit lines in the substrate and a plurality of word lines crossing over the buried bit lines, wherein the substrate under the word lines and between the buried bit lines serves as the channel regions of the memory cells. A method for programming a Mask ROM comprises implanting ions into the channel regions of selected memory cells to raise their threshold voltages, which is called a coding implantation. The data (0/1) stored in a memory cell is dependent on the presence/absence of implanted dopants in the channel region.
In a conventional coding process of a Mask ROM, a photoresist layer is formed on the substrate and then patterned to form coding windows over the channel regions of selected memory cells. An ion implantation is then performed using the photoresist layer as a mask to dope the selected channel regions. However, since the coding windows do not distribute evenly and there must be some regions with dense coding window patterns (dense regions) and some with isolated coding window patterns (sparse regions) on the coding photo mask, the critical dimensions (CD) of the coding windows are not uniform. It is because the optical proximity effect (OPE) for the dense regions is stronger than that for the isolated regions, and the light intensity through the dense regions therefore is higher than that through the sparse regions. The CD deviations of coding windows cause misalignments of the coding implantation, which may results in severe coding errors to lower the reliability of the Mask ROM product.
To prevent CD deviations over the sparse regions and the dense regions, quite a few methods are proposed based on the use of phase shift masks (PSM) or on optical proximity correction (OPC) techniques. The OPC method forms assistant patterns on the photo mask to compensate the CD deviations caused by the optical proximity effect (OPE). However, the two methods both need to design special patterns on the photo masks, so the fabrication of the photo masks are time-consuming, expensive and difficult. Moreover, it is not easy to debug the patterns on such a photo mask after the photo mask is fabricated.
Moreover, a Mask ROM coding implantation is usually performed with boron ions in the prior art. However, the boron dopants implanted into the selected channel regions tend to diffuse laterally to the adjacent buried bit lines, while the boron ions may even be implanted into a portion of the adjacent buried bit lines because of the misalignments or CD deviations of the coding mask. Therefore, the dopant concentrations of the buried bit lines are lowered to cause higher resistance and smaller electric current.